In the technical field of internal combustion engine, various types of variable compression ratio mechanisms have been known. For example, many variable compression ratio mechanisms have been proposed by the present applicant and others, in which a piston top dead center position is displaced vertically by varying a link geometry of a multi-link type piston crank mechanism. Patent documents 1 and 2 describe variable compression ratio mechanisms in which a mechanical compression ratio is varied by vertical displacement of a cylinder with respect to a central position of a crankshaft.
In such a variable compression ratio mechanism, when a drive mechanism for varying the compression ratio is failed, it may become impossible to control the compression ratio, so that the compression ratio may become higher than intended.
As a fail-safe function for such a failure in the drive mechanism, patent document 1 discloses adding a hydraulic circuit for abnormal condition, in a configuration that a cylinder side and a crank case side are configured to be moved relative to each other by a hydraulic drive mechanism. The hydraulic circuit for abnormal condition is configured to guide working oil between two hydraulic chambers, to allow the compression ratio to vary only to a lower compression ratio side, when the hydraulic drive mechanism is abnormal.
Patent document 2 discloses a variable compression ratio mechanism employing an electric motor as a drive mechanism, in which a helical torsion coil spring is provided to apply a force to bias the variable compression ratio mechanism toward the lower compression ratio side. In this mechanism, the biasing force becomes maximized, when the compression ratio is maximized. When the electric motor is failed, the variable compression ratio mechanism is promoted to return to a low compression ratio state by the biasing force of the helical torsion coil spring.
However, according to the configuration of patent document 1, the additional hydraulic circuit for abnormal condition is required, so that the configuration becomes complex, and it is impossible to handle an abnormality in the drive mechanism itself, such as leakage of oil pressure. This configuration is not applicable to cases where the drive mechanism is implemented by another type of mechanism than the hydraulic mechanism (for example, a mechanism employing an electric motor).
On the other hand, according to the configuration of patent document 2 which is biased toward the lower compression ratio side by the spring, when an actuator (electric motor, etc.) drives under a normal condition, the load of driving is large so that the actuator may have to be large-sized, which may adversely affect the fuel efficiency.